A cone type loudspeaker radiates energy from both front and rear surfaces of the cone or diaphragm. As the cone moves forward when energized, a pressure area is created at its front surface and a rarefied area is created at its rear surface. This condition is often referred to as an acoustic doublet, two distinct, out of phase wave patterns traveling away from respective sides of the loudspeaker cone. When the loudspeaker is radiating low frequency (50-400 cps) sound, and the two out of phase wave patterns are allowed to interfere with one another, considerable wave cancellation and poor loudspeaker efficiency result.
To militate against such cancellation, a number of loudspeaker mounting arrangements have been proposed. In what is known as an infinite baffle arrangement, the loudspeaker is mounted in a wall or ceiling between two rooms, the wall or ceiling acting as a baffle which totally isolates the sound waves radiated from the front of the loudspeaker cone from those emanating from the rear of the cone. Such an arrangement is, of course, unsuitable where portability of the loudspeaker is required as in sound amplification equipment used by entertainers. Moreover, in such an infinite baffle arrangement, only the sound waves radiating from one of the cone surfaces are directed at a single listener and, therefore, although cancellation is substantially eliminated, this loudspeaker arrangement is still quite inefficient.
To give portability to an infinite baffle arrangement, loudspeakers have been mounted in sealed enclosures having rigid walls, such enclosures being commonly referred to as acoustic suspension enclosures. Such enclosures are little if any more efficient than the infinite baffle arrangements discussed hereinabove in that sound emanating from the rear of the cone is substantially attenuated within the enclosure. Moreover, such acoustic suspension enclosures, due to the rigid construction thereof have certain charactertistic resonant frequencies associated therewith which render the enclosures limited for use with microphones and amplified musical instruments sensitive to outside sound. When such instruments produce musical tones of such resonant frequencies or harmonics thereof, those tones emitted from the enclosure sympathetically interact with the instrument and are thereby exaggerated in level. This condition is known to entertainers and musicians as "resonating". The parallel rigid walls of the acoustic suspension enclosure allow the maintenance of standing waves therebetween. Such standing waves cause the sympathetic acoustic interaction between the enclosure and a nearby musical instrument or microphone described hereinabove, thereby further rendering the enclosure limited for use with microphones and amplified musical instruments sensitive to outside sound.
Although bass-reflex loudspeaker enclosures, those having an open, tuned port in the loudspeaker mounting baffle, afford some measure of reinforcement of forwardly emitted sound waves by rearwardly emitted sound waves, such enclosures include parallel rigid walls which allow standing waves to be maintained therebetween. Such standing wave maintenance, for the reason set forth hereinabove renders the bass reflex enclosure unsuitable for use with microphones and amplified musical instruments sensitive to outside sound.
In an effort to reduce low frequency enclosure resonance, various prior art loudspeaker enclosures such as those disclosed in U.S. Pat. Nos. 2,713,396 to Tavares and 3,667,568 to Liebscher employ flexible diaphragms or passive radiators in one (usually the rear) enclosure wall. Theoretically, such diaphragms should oscillate in response to sound emitted by the loudspeaker thereby reinforcing the sound emitted from the loudspeaker while altering the compliance of the air within the enclosure sufficiently to reduce and even out enclosure resonance. However, such enclosures have not proven popular for the reproduction of sound from musical instruments. This lack of popularity is due to the inability of known passive radiator materials and structures to tolerate the excursions required for the high amplification of music in the entertainment industry. Furthermore, the rigid enclosure walls employed with the passive radiators allow standing waves to maintain themselves within the enclosure thereby causing a sympathetic interaction between the enclosure and neighboring microphones and/or musical instruments sensitive to outside sound as discussed hereinabove.
Accordingly, it is an object of the present invention to provide a loudspeaker enclosure which overcomes the deficiencies of the prior art.
It is another object of the present invention to provide a loudspeaker enclosure wherein sound waves emanating from opposite sides of a loudspeaker cone carries thereby are constructively combined over a broad range of low frequencies.
It is another object of the present invention to provide a loudspeaker enclosure which prevents the formation of standing sound waves within the interior thereof.
It is another object of the present invention to provide an improved loudspeaker enclosure particularly adapted for use with amplified musical instruments.
It is another object of the present invention to provide an improved musical instrument loudspeaker enclosure which is durable in construction.
It is another object of the present invention to provide a musical instrument loudspeaker which may be efficiently and economically produced.